The present invention relates to a method and apparatus for forming a light absorption film onto an object and, more particularly, to a technology for vapor depositing the light absorption film onto a sensing area of a heat insulating structure such as that of a far-infrared light detector.
There have been many instances where light absorption films for use in photo-sensors are formed of metallic films each in the form of a porous crystalline structure that is grown up in a dendrite phase. In order to form such a metallic film, it has been a general practice to evacuate a vacuum chamber to a desired operating pressure and interrupt evacuating operation whereupon inactive gas is introduced into the vacuum chamber under a desired operating pressure to maintain the same at a constant pressure level. Such an operating pressure is selected to have a value of about several hundreds Pa to cause an average free stroke of reaction series to be extremely shortened. Under such an atmosphere, vapor deposition is carried out using an evaporating boat in a sequence wherein the evaporating boat is resistive-heated to a high temperature and source material composed of metal such as gold is evaporated to form a crystalline, of vapor deposited metal, grown up on a surface of an object (substrate) in a dendrite structure.
In sensors, such as thermopile type far-infrared light detectors, required for detecting a temperature as a thermal energy indicative of an extremely minute incident infrared light, as shown in FIG. 1, a far-infrared light absorption film (heat absorption film) 100 made of gold grown up in the dendrite structure is used, and a cavity 103 is formed directly beneath a sensing area 101 of a supporting substrate 102, with the cavity 103 being utilized to serve as a heat separating structure for thereby providing a remarkable increase in a sensitivity. In such a case, during a stage wherein the vapor deposition film (the far-infrared-ray light absorption film 100) is formed, since the vapor deposition film tends to absorb the infrared ray irradiated from a vapor deposition source, the vapor deposition film becomes extremely sensitive to a film forming temperature and hence a film forming condition must be controlled within a severely narrowed range. The reason for such a severe control is that in a case where the gold light absorption film is deposited over the thermally separated sensing area, the sensing area having an extremely high sensitivity is irradiated with a strong infrared light emanating from the evaporating boat to induce a remarkable increase in temperature. And, if a vapor deposition speed becomes higher than a proper level, then the light absorption film is liable to be miscible with the crystalline such as that formed in a normal film with a high film density with no formation of the dendrite structure and becomes discolored in an ocher color over an entire area. In contrast, if the vapor deposition speed is lower than the proper level, an extended vapor deposition time period is required for obtaining a desired film thickness, causing a resulting deposited film to be heated for an extended period of time. This causes the dendrite structure to be formed with the crystalline grown up in a coarse particle size and the deposited film becomes discolored over the entire area in the ocher color. Thus, the presence of the vapor deposition speed higher than or slower than the proper level causes an increase in a light reflection rate of the film surface, resulting in deterioration in the light absorption capability.
As set forth above, despite the need to execute operation under a severely controlled management, the resulting deposited film has the dendrite structure and hence a difficulty is encountered in observing to monitor the vapor deposition speed and the thickness film at site during the vapor deposition of the light absorption film. The reason for such a difficulty encountered in monitoring is described below in detail.
Methods for detecting the vapor deposition speed widely employed in vapor deposition steps include a technique for detecting a variation in an oscillating frequency of a quarts oscillating element, with a specific detection device for that purpose being commercially available in the market. To simply describe the principle, the quarts oscillating element remains located in the vicinity of the resulting deposited film. When the operation for vapor deposition is implemented, a vapor deposition film is formed over the quarts oscillating element. The quarts oscillating element oscillates at a resonant frequency that decreases in dependence on a mass of the deposited film, with the variation in the resonant frequency being converted into a corresponding thickness of the vapor deposited film to be monitored. However, since the vapor deposited film formed in a dendrite crystalline has a soft structure, the presence of the vapor deposited film formed on the quarts oscillating element causes the mass of the entire film to be hard to contribute to vibration of the oscillating element. For this reason, there is no correlation between the mass of vapor deposited film and the resonant frequency of the quarts oscillating element, resulting in a difficulty in monitoring a status of vapor deposition.
Further, during the vapor deposition process, a speed control can be theoretically possible by controlling the temperature of the vapor deposition boat. When this takes place, the evaporating boat becomes excessively high temperature and, so, the evaporating boat is made of metal with a high melting point such as tungsten or molybdenum. In this instance, it is extremely hard to carry out the measurement, through the use of a thermocouple without consuming the same, in a stable manner at all times.
In addition, while another expedient can conceivably include a technique for monitoring the operating temperature through the use of a radiation thermometer, a mixture of vaporized source material and inactive gas filled in the vacuum chamber circulates therein by convection during the vapor deposition process, causing the light absorption film to be deposited onto a measurement window of the radiation thermometer with a difficulty encountered in stably measuring the operating temperature.
The evaporating boat forms the reaction series wherein the resistance value of the evaporating boat varies every moment depending on the operating temperature and the amount of molten source material fluctuates in an unstable fashion, resulting in an inability of directly controlling the vapor deposition speed for the reasons set forth above. To address such an issue, a state-of-the-art solution includes a method for manually adjusting electric current flowing through the vapor deposition boat with perception and experience of an operator while observing a status of source material remaining on the vapor deposition boat and a status of vapor of a vaporized product in a peripheral area of the source material to determine conditions under which the vapor deposition process is carried out. For this reason, it was extremely difficult to stably control and realize a desired film forming condition.
The present invention has been completed to address the above identified issues encountered in the state-of-the-art, and it is an object of the present invention to provide a method and apparatus for forming a light absorption film to realize a desired film forming condition in a stable and easy manner.
The inventors of the present patent application has uncovered a fact that the applied electric power correlated with the variation in the vapor deposition speed has a value in a deviation range of approximately xc2x15% under a condition to enable a desired film colored in from black and brown to black colors to be obtained when forming the far-infrared light absorption film.
With the above view in mind, according to an aspect of the present invention, there is provided a method for forming a light absorption film, composed of a porous metallic vapor deposited film with crystalline grown up in a dendrite structure, in a vacuum vapor deposition, comprising: preliminarily measuring and storing a time profile of targeted electric power in a storage unit by preliminarily applying the electric power to an evaporating boat to perform resistive-heating to measure electric current, flowing through the evaporating boat, and a voltage potential, appearing at both sides of the evaporating boat, that enable a proper film quality to be realized; and controlling current electric power, that is a product of the electric current currently flowing through the evaporating boat and the voltage potential currently appearing at the both sides of the evaporating boat during a current vapor deposition, so as to match the time profile of the targeted electric power stored in the storage unit.
With such a structure, the electric power to be applied is precisely controlled for properly controlling the vapor deposition speed, thereby enabling the desired light absorption film to be obtained at all times.
According to another aspect of the present invention, there is provided a method for forming a light absorption film, composed of a porous metallic deposited film with crystalline grown up in a dendrite structure, in a vacuum vapor deposition, comprising: preliminarily measuring and storing a targeted time changing rate of a combined resistance, in a storage unit, between an evaporating boat and source material obtained by preliminarily applying electric power to the evaporating boat to perform resistive-heating to measure electric current, flowing through the same, and a voltage potential, appearing at both sides of the evaporating boat, that enable a proper film quality to be realized; and controlling electric power to be applied to the evaporating boat such that a current time changing rate of the combined resistance obtained by the electric current currently flowing through the evaporating boat and the voltage potential currently appearing at the both sides of the evaporating boat during a current vapor deposition matches the targeted time changing rate of the combined resistance stored in the storage unit.
According to another aspect of the present invention, there is provided a light absorption film forming apparatus, comprising: a vacuum vapor deposition device having an evaporating boat to perform resistive-heating; a storage unit storing a time profile of targeted electric power obtained by measuring electric current flowing through the evaporating boat and a voltage potential appearing at both sides of the same that enable a proper film quality to be realized; and a control unit controlling current electric power to be applied to the evaporating boat such that the current electric power obtained by a product of the electric current currently flowing through the evaporating boat and the voltage potential currently appearing at the both sides of the same during a current vapor deposition matches the targeted time profile of the applied electric power stored in the storage unit.
According to a further aspect of the present invention, there is provided a light absorption film forming apparatus, comprising: vacuum vapor deposition means having an evaporating boat to perform resistive-heating; storage means for storing a time profile of targeted electric power obtained by measuring electric current flowing through the evaporating boat and a voltage potential appearing at both sides of the same that enable a proper film quality to be realized; and control means for controlling current electric power to be applied to the evaporating boat such that the current electric power obtained by a product of the electric current currently flowing through the evaporating boat and the voltage potential currently appearing at the both sides of the same during a current vapor deposition matches the time profile of the targeted electric power stored in the storage unit.
According to a further aspect of the present invention, there is provided a light absorption film forming apparatus, comprising: a vacuum vapor deposition device having an evaporating boat to perform resistive-heating; a storage unit storing a targeted time changing rate of a combined resistance between the evaporating boat and source material obtained by preliminarily applying electric power to the evaporating boat to perform resistive-heating to measure electric current, flowing through the same, and a voltage potential, appearing at both sides of the evaporating boat, that enable a proper film quality to be realized; and a control unit controlling electric power to be applied to the evaporating boat such that a current time changing rate of the combined resistance obtained by the electric current currently flowing through the evaporating boat and the voltage potential currently appearing at the both sides of the evaporating boat during a current vapor deposition matches the targeted time changing rate of the combined resistance stored in the storage unit.
According to a still further aspect of the present invention, there is provided a light absorption film forming apparatus, comprising: vacuum vapor deposition means having an evaporating boat to perform resistive-heating; storage means for storing a targeted time changing rate of a combined resistance between the evaporating boat and source material obtained by preliminarily applying electric power to the evaporating boat to perform resistive-heating to measure electric current, flowing through the same, and a voltage potential, appearing at both sides of the evaporating boat, that enable a proper film quality to be realized; and control means for controlling electric power to be applied to the evaporating boat such that a current time changing rate of the combined resistance obtained by the electric current currently flowing through the evaporating boat and the voltage potential currently appearing at the both sides of the evaporating boat during a current vapor deposition matches the targeted time changing rate of the combined resistance stored in the storage means.